Field
Embodiments described herein generally relate to catalytic gasification of coal. More specifically, embodiments described herein relate to coal gasification with an FeCO3 catalyst.
Description of the Related Art
Integrated gasification combined cycle (IGCC) of coal offers the advantages of higher efficiency and capability of CO2 and pollutant separation compared to conventional coal-fired power plants. One important step in IGCC is coal gasification, which occurs within an energy-intensive high temperature environment. Therefore, the power generation industry and other natural resource industries are increasingly interested in using catalysts to improve coal gasification. Various catalysts for use in coal gasification have been investigated, but many are often cost prohibitive on an industrial scale. Iron compounds are potential gasification catalysts due to their abundance, low cost, and environmentally friendliness. Iron compounds have been previously investigated to catalyze coal gasification and their effects on coal pyrolysis and char gasification as well as tar formation during the whole coal gasification process.
Ohtsuka et al. used X-ray diffraction (XRD) to investigate the effect of three iron compounds on coal pyrolysis. When using FeCl3 as the iron precursor, Ohtsuka et al. reported the presence of both metallic a-Fe and FeO (wustite) when devolatilization was conducted in an inert atmosphere, and existence of Fe3O4 (magnetite) when devolatilization was performed within a steam environment. The effect of Fe2(SO4)3 was also studied and similar results were obtained, except for the appearance of FeS peaks within both inert and steam devolatilization environments. When Fe(NO3)3 was employed as the iron precursor in an inert atmosphere, a-Fe (crystal size <30 nm) and Fe3C were detected, while a combination of small crystallites (<10 nm) of Fe3O4 with FeO were found when steam was used for devolatilization. In addition, Ohtsuka et al. found that Fe(NO3)3, transformed into fine particles of mixed iron oxides during devolatilization, was effective for steam gasification while chloride and the sulfate, converted to magnetite with a large crystallite size, were not effective.
Alternatively, Song and Kim reported that Fe(NO3)3 was less active than FeSO4 during pyrolysis of a sub-bituminous coal with the same iron loading (3 wt %) at 700-800° C. Yu et al. loaded iron onto a brown coal using FeCl3 solution and obtained metallic α and γ Fe in the pyrolysis step and magnetite during gasification. Domazetis et al. reported that the char formed during pyrolysis of brown coal with added iron contained iron oxides and carbonates, as measured by X-ray photoelectron spectroscopy (XPS).
Coal tar is one of the byproducts formed during coal gasification. Tromp et al., using gas-chromatography (GC) and gas-chromatography-mass spectrometry (GC-MS), identified that the volatile compounds from a pyrolysis process included varying amounts of polycyclic hydrocarbons. Zeng et al. studied the tar and the soot generated by tar's secondary reactions, using Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy of the dissolved samples in tetrahydrofuran (THF), also found that the tar molecules were typically poly-aromatic hydrocarbons (PAH) substituted with functional groups or heteroatoms, such as alkyl chains, oxygen, nitrogen, etc. Iron containing materials have also been tested for their effects on tar formation during coal gasification. Cypres and Soudan-Moinet found that iron oxides (Fe2O3 or Fe3O4) reduced both the primary devolatilization rate of coal (between 300° C. and 600° C.) and the tar and gaseous hydrocarbon yields, while the composition of the investigated tar did not change. Cypres and Soudan-Moinet also found that hematite iron had a greater influence than magnetite iron. Moreover, limonite (FeO(OH).nH2O) of various origins, iron oxides (FeO, Fe2O4, Fe3O4, and Fe2O3), ankerite (CaFe(CO3)2), sintered iron ore, and pelletized iron ore were evaluated by Nordgreen et al. and concluded that metallic iron from pre-reduced hematite (Fe2O3) was an effective catalyst and achieved almost 100% tar decomposition at 900° C.
Different mechanisms have been proposed to explain the effect of iron catalysts on the gasification of carbonaceous materials, including coal. Matsuoka et al. studied steam reforming of woody biomass in a fluidized bed at 500-700° C. with an Fe/γ-alumina catalyst and suggested that redox reactions take place on the iron oxide surface. Yu et al., Hermann and Huttinger, and Xu et al. indicated that the overall reactions with non-catalyzed steam carbon gasification occur on carbon particles and they are relatively simple, while those with iron-based steam gasification mainly proceed on the surface of iron species and much more complicated.
As described above, iron-based catalysts may be advantageous in coal gasification processes. However, certain iron salts may be cost prohibitive on an industrial scale. In addition, SO42−, NO3−, and Cl− moieties may damage gasification equipment and also harm the environment. Moreover, H2, CO, and CO2 yields may be less than desirable depending on the catalyst utilized.
Thus, what is needed in the art are improved coal gasification catalysts and methods of utilizing catalysts in coal gasification processes to improve conversion and minimize deleterious effects of by-products.